Persistent-mode operation of a cryocooler-cooled HTS magnet

Persistent-mode operations of a cryocooler-cooled HTS magnet were carried out. The closed circuit of the magnet consisted of a Bi-2212 solenoid coil and a NbTi/CuNi persistent-current switch (PCS). Thermal conductance between the PCS and the cooling stage was designed not to heat up the coil. The temperature increase was maintained within 0.1 K during the normal state of the PCS. The transition times of the PCS between a superconducting and a normal state were sufficiently short for practical use. The magnet started a persistent-mode operation from an initial magnetic field of 2.1 T. Decay behaviors during persistent-mode operations were successfully observed over 90 hours. Except for the initial period, field decay was limited by resistance of the joint. A method to compensate the field decay was performed by installing a NbTi cylinder in the coil bore.